1. Field of the Invention
The present invention relates to an optical switch system that switches an optical signal entered from an input port by using micro mirrors such as MEMS (Micro Electro Mechanical Systems) mirrors so as to send out the optical signal to an output port. For example, the present invention is used for OXC (Optical Cross-Connect) system with a WDM (Wavelength Division Multiplexing) communication or a space division communication or the like.
2. Description of the Prior Art
Conventionally, there has been developed micro mirrors utilizing an MEMS technology called a DMD (Digital Micro-mirror Device), which is used for an optical switch system that is disposed at a node of an optical network. The optical switch system includes a plurality of micro mirrors arranged in a plane, and each of the mirrors has a reflection plane whose angle can be controlled. Optical signals entered from a plurality of input ports are reflected by the plurality of micro mirrors, and each of the reflected optical signals enters a selected output port among the plurality of output ports. In this way, ports are switched so that an optical exchange of a plurality of optical signals on an optical transmission channel can be performed.
Concerning a control of such micro mirrors in the optical switch system, following techniques are disclosed conventionally.
Japanese unexamined patent publication No. 2003-15123 discloses a liquid crystal display device of a reflection type, which is made up of a liquid crystal display panel of a transmission type and micro mirrors arranged at the rear side of the liquid crystal display panel. This device adjusts a luminance of transmitted light from the transmission liquid crystal display panel by a reflection angle of a micro mirror. More specifically, an angle of a micro mirror plane is altered with respect to the optical axis of irradiation light so as to adjust a luminance variation of transmitted light of each pixel in the LCD. In addition, a luminance level of transmitted light from each pixel in the LCD is corrected for each pixel, so that a flicker adjustment, a cross talk adjustment or a ghost adjustment is performed.
In addition, Japanese unexamined patent publication No. 2005-99682 discloses a technique of controlling an angular position of a micro mirror by a closed loop using a mirror angle sensor for detecting an angular position of the micro mirror.
An optical switch system performs an angular control of micro mirrors in which the micro mirror is controlled to be an appropriate angle so that light from an input port is sent to an output port. Inherently, a theoretical value of a control angle can be calculated from a geometric arrangement of the micro mirror, the input port and the output port. In reality, however, the control angle may be deviated from the theoretical value due to a structural variation of the micro mirror or an assembly error of an optical system lens or the like. Therefore, a control angle (i.e., a real measurement optimum point) or a VOA (Variable Optical Attenuator) angle (i.e., a real measurement VOA point) is measured, which produces a least loss even if the output port is connected to an optical monitoring device or the like.
On the other hand, the micro mirror has an advantage in that it can be downsized. For this reason, however, it has tendency to have an electrical cross talk between neighboring mirrors. This cross talk may affect the control angle of the micro mirror and cause an angle shift. If an angle shift is generated in the micro mirror, an optical coupling efficiency between the input port and the output port is decreased so that an insertion loss may increase.
However, it is difficult to reduce the influence of the cross talk between micro mirrors, so any adjustment or correction is not performed for that purpose.
Further, in the technique disclosed in the second document described above, the mirror angle sensor detects an angular position of the micro mirror, and the angular position of the micro mirror is controlled by the closed loop. Therefore, an influence of the cross talk is substantially reduced. However, mounting such a mirror angle sensor in each micro mirror may cause an increase of a mounting area and a cost, which is disadvantageous for a compact size and a low cost.